A photopolymerizable composition is principally composed of a photopolymerizable initiator and a polyfunctional monomer. Upon irradiation with light, the photopolymerizable composition is hardened and insolubilized in a solvent. Utilizing this property, photopolymerizable compositions have been widely utilized in photography, printing, metal surface finishing, ink, etc. (J. Kosar, Light Sensitive Systems, 1965, published by J. Wiley & Sons, New York).
Heretofore investigations have been made to improve the sensitivity of a photopolymerizable composition to light and many photopolymerization initiators have been proposed. Examples of such photopolymerization initiators include benzoin ethers described in U.S. Pat. No. 2,722,512; anthraquinones described in U.S. Pat. No. 3,046,127; aminophenyl ketones and active methylene or amino compounds described in Japanese Patent Publication 11936/74 (corresponding to U.S. Pat. No. 3,661,588); Michler's ketones and benzophenone described in U.S. Pat. No. 3,682,641; benzophenone and 4-N,N-dimethylaminobenzophenone described in Japanese Patent Publication 16725/75; etc. When these photopolymerization initiators are employed, the improvement in light sensitivity is indeed noted but, dynamic properties of the resulting light hardened matters (particularly hardened matters in a layer form) were not alway satisfactory for various utilities.
For example, in the case of using the hardened matter as a dry film resist employed in preparation of a printed circuit base plate, the layer strength after photohardening was insufficient. Dry film resists employed for preparing printed circuit base plates are described in Japanese Patent Publication 25231/69 (corresponding to U.S. Pat. No. 3,469,982) and more detailed use is described in, e.g., W. S. De Forest, Photoresist, pp. 163 to 212 (1975), published by McGraw-Hill, New York. A main object of using a dry film resist is to prepare a through hole by tenting but in the case of conventional known photopolymerization initiators, troubles of breaking layers at steps of development and etching often occur due to insufficient strength of a tenting layer.